The out-of-plane intercalate phonons of superconducting YbC6 have been
measured with inelastic x-ray scattering. Model fits to this data, and
previously measured out-of-plane intercalate phonons in graphite intercalation
compounds (GICs), reveal surprising trends with the superconducting transition
temperature. These trends suggest that superconducting GICs should be viewed as
electron-doped graphite.